[考点] 细节的理解
[解析]
1.做题分析: ①What causes a helix in nature to appear with
either a dextral (right-handed, or clockwise) twist or a sinistral (left-handed, or counterclockwise) twist is one of the most intriguing puzzles in the science of form. ②Most spiral-shaped snail species are predominantly dextral. ③But at one time, handedness (twist direction of the shell) was equally distributed within some snail species that have become predominantly dextral or, in a few species, predominantly sinistral. ④
What mechanisms control handedness and keep left-handedness rare? ⑤
It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images,
for any disadvantage that a sinistral twist in itself could confer on its possessor
is almost inconceivable. ⑥But left- and right-handed snails are not actually true mirror images of one another. ⑦Their shapes are noticeably different. ⑧
Sinitral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features. ⑨In addition,
perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions. ⑩
Presumably an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and left-handed populations.
But this evolutionary mechanism combining dissymmetry, anatomy, and chance
does not provide an adequate explanation of why right-handedness should have become predominant.
It does not explain, for example, why the infrequent unions between
snails of opposing hands produce fewer offspring of the rarer than the commoner form in species where
each parent contributes equally to handedness.
Nor does it explain why, in a species where
one parent determines handedness, a brood is not exclusively right- or left-handed when the offspring would have the same genetic predisposition.
In the European pond snail Lymnaea peregra, a predominantly dextral species whose handedness is
maternally determined, a brood might be expected to be exclusively right- or left- handed - and this often occurs.
However, some broods possess a few snails of the opposing hand, and in predominantly sinistral broods, the incidence of dextrality is surprisingly high.
Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness.
In the case of Lymnaea peregra, studies indicate that a dextral
gene is expressed during egg formation; i.e., before egg fertilization, the gene produces a protein, found in the cytoplasm of the egg, that controls the pattern of cell division and thus handedness.
In experiments, an injection of cytoplasm from dextral eggs changes the pattern of sinistral eggs, but an injection from sinistral eggs does not influence dextral eggs.

One
explanation for the differing effects is that all Lymnaea peregra eggs begin left-handed but most switch to being right-handed.

Thus the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.
2.文章结构 本文依然是以问句为主题句的文章。
对于没有经过上课,或者对GRE阅读方法不熟悉的考生,这篇长阅读很难且专业性很强。本文不仅出现了很多我们陌生的术语,很多作为细节的句子也比较复杂。
在阅读长文章时必须从整体入手,以每段为单位,时刻抓住主题,并判断每一段讲的内容是如何服务主题的。明确了段与段之间的关系后,大概扫一下每段内的句子,如果是理论,则要确定理论的功能,可以大概扫一眼关键词,以便做题时方便定位;如果是事例,只需要看一下事件的主角即可,内容不要看。总之,做长阅读时必须分清主次。
文章第一段先交代背景,提出问题作为主题:什么控制蜗牛螺旋方向(手性)?第二段第一句回答了这个问题:如果认为左手性和右手性是完全对称的,进化不能解释生物手性的问题,后面的内容都在解释这一点;第三段第一句再次回答这个问题:即使考虑了不对称性,进化还是不能解释生物手性的问题,作者用了几个自然中的例子来证明这一点;最后一段提出发育理论可能可以解释生物手性的问题,用了科学研究和实验室的例子来证明这个观点。
3.文章精析 ①提出一个令人费解的现象:蜗牛的旋性到底是由什么引起的。预判文章后面可能会据此提出各种理论或假设来解释清楚这一个问题。
②提出一个事实,依然没有看到针对前面的现象提出的解释,继续往后看。
③But表转折,提出另外一个事实,依然没有看到针对前面的现象提出的解释,继续往后看。
④出现问句,且为主题句。提出两个问题:第一,到底是什么机制(原理)来控制蜗牛这种旋性的?第二,到底是什么机制导致左旋那么稀少?预判文章后面的内容,一定是在回答这两个问题的。
⑤这一句看起来特别复杂,其实很简单。首先当看到evolution的时候,不要立刻习惯性想到“进化论”,而是要想到这里的evolution其实是前面第四句主题句里面的其中一种mechanism,该句中的unlikely为否定逻辑,也就是这一句作者认为,如果左旋与右旋完全呈镜像对称,那么不可能是evolution来控制这种旋性的。简而言之,evolution不是前面问句的答案。
⑥出现了But,说明evolution是有可能的。GRE阅读难就难在这里,一段话不断出现各种让考生意想不到的逻辑关系,绕晕考生。
⑦依然在讲它们是不同的,细节,略读。
⑧出现因果逻辑关系,这一句的意思是:左旋稀少,可能是某种缺陷带来的结果。这一句其实是在回答第一段的第二个问题。缺陷后面的定语可以先略读。
⑨出现并列逻辑,表明这一句与前一句为并列逻辑关系,也就是左旋少的原因之一:它们无法交配。
⑩这一句告诉了我们,让左旋稀少的原因还有,本来稀少的旋性的蜗牛很难找到对象,因此导致其更加稀少,或者产生了地理隔离。

出现了But,表转折,且为主题句。这一句又开始说进化论,还有前面提到的各种原因,都无法解释清楚为什么左旋那么稀少。看到这里,不少读者已经彻底晕了,整篇文章一会儿说evolution不可以,一会儿又说可以,接着下一段又说不可以。读者还需要额外注意的是,该句中的combining表并列逻辑,其后的dissymmetry, anatomy, and chance分别与前面的几个原因一一对应:为镜像不对称;后天伴随而来的缺陷;很难找到适合的对象。

出现for example,举例证明该段的第一句主题句,细节,略读。

前一句表并列,证明该段的第一句主题句,细节,略读。

举例证明该段的第一句主题句,细节,略读。

与前一句表转折,但依然是用来证明该段的第一句主题句,细节,略读。

为本段的主题句。出现了发育理论(developmental mechanism),需要注意的是,当看到这个机制,依然要想到的是:这个机制也是为了回答原文第一段的那个问句(主题句),只是这里讲到进化论必须让位给发育理论,说明这一段应该是两个理论之间的关系,进化论解释不了本文的问题,但是发育理论可以解释清楚。

举例子证明本段的主题句,略读。

用实验证明这一段的主题句,略读。

依然是实验的内容,细节,略读。

全文总结:解决这个问题的道路也是曲折的。
题型:细节内容改写题
根据题目“concomitant structural features”可以快速定位到原文的第八句。根据上面对文章的分析得知,答案应该要符合的特点是:由伴随而来(先天)的结构带来的缺陷。此题为具体与抽象的替换。
A选项:没有说到缺陷,所以是错的。
B选项:没有说到结构缺陷,所以是错的。
C选项:这个chip或者fracture是后天造成的结构,而且也没有提到缺陷,所以错。
D选项:看到better就知道这个选项是错的了,尽管不知道camouflages是什么意思。
E选项:是对的。smaller shell opening与原文的concomitant structural features为具体与抽象的替换;restricts mobility and ingestion与原文的disadvantage为具体与抽象的替换。